Externally illuminated sign boards such as billboards or tradesmen's store front signs, with flood lights or foot lights supplying direct illumination to the sign face, have been conventional since the introduction of electric lights. Overhead floodlighting illumination of such signs is illustrated in Monheim, U.S. Pat. No. 1,871,073, issued in 1932 and Frederick, U.S. Pat. No. 1,735,040, issued in 1929.
Backlighted translucent signs are equally conventional, as shown in Willey, U.S. Pat. No. 1,724,243, issued in 1929. More recently, edge lighted signs have been proposed, as shown in Buc, U.S. Pat. No. 3,546,438, issued in 1970; Mellyn, U.S. Pat. No. 3,402,493, issued in 1968 and Decaux, U.S. Pat. No. 4,249,231, issued in 1981.
Overlying floodlighting bulbs illuminating flat reflector surfaces are shown in the 1929 Frederick U.S. Pat. No. 1,735, 040 and also in the 1933 Harvey U.S. Pat. No. 1,915,666. Light bulbs arrayed along the top edge of an opaque sign are shown in Kettles U.S. Pat. No. 802,646, issued in 1905, which also shows a similar array of bulbs along both the top and bottom edges and along all four edges of a rectangular sign embraced by outlying curved reflectors of semi-cylindrical shape, with light rays being thrown equally on opposite sides of the sign board. Such semi-cylindrical reflectors approximate a parabolic cylinder in shape, and tend to gather and converge entering parallel light rays by reflecting them toward a focal line, rather than diffusing them over the face of an opaque sign plaque.
A polygonal array of flat mirror surfaces, directing a series of flashing light spots across the face of a sign board is shown in Monheim, U.S. Pat. No. 1,871,073, issued in 1932.
Conventional opaque illuminated sign plaques have suffered from the common fault of uneven illumination, being brightly lit near the bulbs along one edge of the sign plaque and unlit or dimly lit along the opposite edge. Indeed the 1905 Kettles patent suggested illumination along all four edges of a rectangular opaque sign plaque, in an effort to overcome this unequal illumination problem, and the resulting energy costs for illuminating such a sign make it highly uneconomical.